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The Effect of AC Bias Waveform on Harmonic Distortion in Magnetic Tape Recording

The effect of a.c. bias waveform on harmonic distortion in magnetic tape recording is studied analytically and experimentally. An analysis of square wave bias is performed, following methods used by Camras and Zenner, using the same power series approximation to a Br-H curve as used by Zenner. Comparing these results to those obtained by Zenner for sinusoidal bias, square wave bias is found to result in higher output and higher percentage distortion. The analysis is extended to non-symmetrical rectangular bias, for which the distortion is found to be much greater. The experiments are performed by generating various voltage waveforms at 20 K and feeding them through a current feedback amplifier to a record head, where they serve as bias and are superimposed on 1 KC sinusoidal audio signals. The results indicate little difference in distortion between sinusoidal and square wave bias, but greatly increased distortion for any type of unsymmetrical bias. A comparison is made between the analytic and experimental results in an attempt to evaluate two different theories on the mechanism of a.c. bias. It is concluded that both theories are valid but for different relationships between gap length and bias wavelength.